In such a machine, the rail is fitted at its ends, either by fastening, or by wedging, or by guiding without play, on vertical columns.
The movable machining head, which can be a sliding drill head, or a boring slide, or a milling carriage, or a lathe slide, is generally guided by sliding ways placed on the front face of the rail.
During the displacement of the machining head, the rail becomes deformed, on the one hand, by bending since it has to support the weight of the machining head, and on the other hand, by twisting since it is subjected to a torsional couple which is equal to the product of the weight of the machining head and the distance separating the axis of the rail from the line of application of said weight. Such deformation affects the accuracy of the displacement of the machining head on the rail.
It is known that the deformation due to the torsional couple can be eliminated by fitting s balance weight which is fixed to the machining heed and placed on the other side of the rail, but the disadvantage of this solution is that it increases the weight of the machining head, increasing as s result the deformation due to bending.